Use any Auth Method via annotation

[pbar1]

Is your feature request related to a problem? Please describe.
Out of the box, the Vault Agent Injector does not support other Auth Methods than the kubernetes method using annotations alone.

Describe the solution you'd like
It would be great to be able to configure the Auth Method of the Agent that gets injected based on annotation alone. For example, such annotations as:

vault.hashicorp.com/auth-type: cert             # new annotation
vault.hashicorp.com/auth-path: auth/cert
vault.hashicorp.com/auth-config-name: pki-test  # new annotation
vault.hashicorp.com/ca-cert: /certs/ca.pem
vault.hashicorp.com/client-cert: /certs/client.pem
vault.hashicorp.com/client-key: /certs/client-key.pem

Should be able to generate config that looks like this:

{
  "auto_auth": {
    "method": {
      "type": "cert",
      "mount_path": "auth/cert",
      "config": {
        "name": "pki-test"
      }
    },
    "sink": <omitted for brevity>
  },
  "exit_after_auth": false,
  "pid_file": "/home/vault/.pid",
  "vault": {
    "address": "https://vault.vault.svc.cluster.local:8200",
    "ca_cert": "/certs/ca.pem",
    "client_cert": "/certs/client.pem",
    "client_key": "/certs/client-key.pem",
  },
  "template": <omitted for brevity>
}

Describe alternatives you've considered
The obvious alternative is using the vault.hashicorp.com/agent-configmap annotation to configure the agent from an existing ConfigMap. However, this solution requires the ConfigMap in question to already exist in order to be consumed - to use it in a preexisting Helm chart for example, one would have to modify the chart to explicitly create the ConfigMap explicitly. This isn't very declarative and violates the decorator-like pattern of annotations if used extensively.

Additional context
I've submitted a pull request extending the existing code to support this feature, #213. Let me know how I can help to get this feature merged. More details are in that PR.

[pbar1]

Fixed by #213!